The ability to detect minute amounts of protein is of interest both in protein chemistry and immunology. A possible replacement for radioimmuno assay (RIA) is the tagging of the protein by a caged molecule of Europium. By taking advantage of the very long fluorescence lifetime of Europium (500 microseconds) extreme sensitivities are possible. A pulsed nitrogen laser operating at 337 nm at 15 pps and power output of 200 microjoules has been used with a 1/2 inch end on PET photomultiplier tube to examine a simple Europium tagged system. Detection of 10-14 molar Europium was readily achieved in a 100 microliter sample. Efforts to extend this to 10-16 M are presently being pursued along with techniques for greatly improving reliability of both the phototube, counting electronics, and the laser light source. One of the possible uses of such a system is in the detection of very low levels of retrovirus.